The invention relates to a method for milling deep slots for producing a plurality of walls, parallel to one another, on a workpiece, of the needed for instance to produce forked roots of turbine blades, and to an appropriate milling tool for the purpose.
Larger turbines, such as steam turbines, have a rotor that carries many blades. They are retained in corresponding slots provided in the rotor shaft. One structural form of the turbine roots is known as forked roots. These forked roots have a plurality of walls substantially parallel to one another, between which deep slots are formed. With these walls, also known as fingers, the blade root is seated in corresponding slots of the rotor shaft.
In a prior art production method for the blade roots, the slots are first preroughed in a first work station with a milling tool that is formed by a row of HSS disk milling cutters joined together in a manner fixed against relative rotation and seated on a common shaft. Cutting speeds of 20 m/min are attainable, which is equivalent to feeding speeds of 10 to 12 mm/min. For a blade root whose slots can have a depth of between 50 mm and 120 mm, the result is a machining time solely for opening the slots that is up to ten minutes long.
In a second work step, the preroughed slots are then smoothed with another milling tool. This milling tool has a row of disk-like smoothing milling cutters held on a shaft. The two operations are performed in separate stations that the turbine blades pass through in succession. There is a need to shorten the machining time.
From German Patent No. 44 31 841 (corresponding Gauss et al. U.S. Pat. No. 5,676,505), milling of deep slots in generator rotors or turbine rotors in a two-step method is known. The turbine rotor has relatively deep slots that serve to receive windings. The slots are each opened individually with a disk milling cutter that has rough cutters on its circumference and smooth cutters on its sides. Although here the slots are open in two steps, the rough machining and the smooth machining are combined in each step.
The length of the slots exceeds their depth by far. Thus parts of the slot wall located ahead of the milling cutter and behind the milling cutter can stabilize the slot wall that is being milled at the moment. Moreover, each slot wall is machined on only one flank.
The object of the invention is to create a method for milling deep slots for producing a plurality of walls parallel to one another in a workpiece that has a shortened machining time. In particular, the method should be suitable for producing forked roots of turbine blades.